The advent of the wireless communication era has brought about the evolution of a variety of air interface standards that define a plurality of wireless communication protocols. Such wireless communication protocols support standards as defined by the third generation partnership project (3GPP), which is a collaboration between groups of telecommunications associations that seek to develop a globally applicable, third generation (3G) mobile phone system specification within the scope of the International Mobile Telecommunications 2000 project of the International Telecommunication Union (ITU). A sampling of relevant 3GPP specifications include long-term evolution (LTE), worldwide interoperability for microwave access (WiMAX), wideband code division multiple access (WCDMA) and time-division synchronous code-division multiple access (TD-SCDMA) specifications to name only a few.
3G wireless communication systems employ, among other features, complex modulation formats to transmit voice, data, and multimedia services over limited bandwidth channels. Such wireless communication systems offer high performance with dependable reliability, but depend upon the linearity of the power amplifiers that exist within the final transmitter stages. Stated differently, the wireless communication systems depend upon their power amplifiers to boost the amplitude of signals that exhibit complex modulation formats without excessive distortion and without excessive signal power leakage outside of the designated frequency band of transmission.
Signal power leakage into adjacent transmission channels, also known as adjacent channel leakage (ACL), may create enough distortion in adjacent channels so as to create excessive bit errors within those channels. As a result, maintaining an adequate ACL ratio (ACLR) throughout the dynamic range of the power amplifier is a critical component that is required to maintain the bit error rate (BER) within acceptable limits.
ACLR, however, may be adversely affected by imperfect in-phase/quadrature-phase (I/Q) modulated spectra that exists within the power amplifier's input signal. ACLR may also be adversely affected by out-of-channel carrier noise and/or intermodulation distortion that may be added by the power amplifier itself. In particular, as the power amplifier approaches compression, the power level of the resultant out-of-band spectral components, or intermodulation (IM) products, increases as well. Since the frequencies of the out-of-band spectral components often exist within the frequency band of adjacent channels, ACLR may be adversely affected.
For WCDMA systems, ACLR is defined to be the ratio of the integrated signal power in the adjacent channel to the integrated signal power in the main channel. Accordingly, an increase in the ACLR is indicative of a communication system that may exhibit increased BER with a resultant decrease in system performance. Unfortunately, WCDMA systems utilize communication signals that exhibit relatively high peak-to-average power ratios, or crest factors. As a result, a power amplifier that is operating close to compression, but is nevertheless operating within a relatively linear region, may be driven into compression with a resultant increase in ACLR due to the high crest factors of the signals being amplified.
One technique that may be used to minimize non-linear operation of a power amplifier of a wireless communication system is simply to reduce the drive level of the power amplifier by an amount that is substantially less than the anticipated peak-to-average power ratio. Such a decrease in the signal drive level, however, may compromise the radio link performance, since a decrease in signal drive level often results in a corresponding reduction in signal-to-noise ratio (SNR). Increasing the signal drive level in order to increase SNR, on the other hand, may lead to gain compression of the power amplifier, which increases the IM products generated by the power amplifier, thereby increasing ACLR as discussed above.
Accordingly, selecting the appropriate drive level of a power amplifier within the final transmitter stage of a wireless communication system is critical to minimize ACLR while maximizing SNR. However, selecting the appropriate drive level is a complicated task, since the compression point of the power amplifier may change over time and temperature. In addition, the peak-to-average power ratio of the transmitted waveform may be highly unpredictable, which further complicates the appropriate selection of the power amplifier's drive level.
Efforts continue, therefore, to develop dynamic and predictive techniques that facilitate appropriate selection of the drive level of power amplifiers that are utilized within wireless communication systems.